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Microorganisms (bacteria and fungi inclusive) produce definite patterns on culture media plates as they grow and divide. These specific patterns (inclusive of shapes and sizes) also aid in the preliminary identification of the organisms for further characterization and identification. Colonial morphology is the size, shape, colour, texture and the general structure of an individual colony of a particular microorganism (in this case bacterium or fungus) on a culture media plate that supports its growth. It describes the attributes of an individual colony of microbe (for example, fungi or bacteria) that is growing on a culture medium that supports its growth. These specific features (as outlined in Table 1) which are unique to bacteria and fungi are used for the preliminary identification of a bacterium or fungus in the microbiology laboratory.


Table 1. Schematic illustrations of the colonial/cultural characteristics of bacteria on solid media

Colonial morphology which can also be referred to as cultural characteristics refers to the macroscopic appearance of a bacterium on different kinds of growth (culture) media. A microscope is usually not required to establish the cultural characteristics of a bacterium, rather, the visible eyes is mostly used for the identification. Therefore, the observations are usually made macroscopically with the naked eyes. However, microscopical examination of colonial characteristics of a bacterium may be necessary, for example, when examining the margin/edge of a bacterium in order to decipher if it is actually smooth or filamentous. Colonies from the culture media plate can also be stained and viewed microscopically for further observation and characterization. Aside this, a wet mount preparation from the colonies on the culture media plate can also be undertaken and viewed microscopically to determine some unique features of the organism such as motility.

A colony is a macroscopically visible population of cells growing on solid culture media, and that arises from a single cell. Microorganisms (particularly bacteria and fungi) are presumptively recognized on solid agar media or plates based on some unique growth features which they express on such growth or culture media (Figure 1). Microorganisms (bacteria and fungi) grow as colonies on solid culture media; and this helps in their preliminary identification and characterization. The growth of a bacterium or fungus in a solid growth media is thus taken into consideration when characterizing the isolated organism. The colony of an organism is visibly studied when establishing the cultural characteristics of a bacterium or fungus. Table 1 shows the colonial/cultural characteristics of microorganisms (bacteria or fungi) on solid culture media plates.

Figure 1. A culture of Klebsiella pneumoniae growing on MacConkey (MAC) agar plate. The organism produces mucoid and roundish colony on MAC agar medium.Photo courtesy: https://www.microbiologyclass.com

The appearance of bacterial or fungal colonies on a culture media plate is generally species-specific and can be very helpful in identifying isolates on individual basis. The size of a bacterium can be measured using a meter rule to determine its diameter; and this can be achieved using the microscope or any other advanced method of microbial measurement. Bacterial colonies can vary from large colonies to tiny colonies that are less than 1 mm in diameter. A bacterium that is less than 1 mm in size is generally called punctiform (i.e., pin-point). The appearances of some observed colonial growth of a bacterium on a solid culture medium plate are as outlined in Table 1.

Further reading

Brooks G.F., Butel J.S and Morse S.A (2004). Medical Microbiology, 23rd edition. McGraw Hill Publishers. USA.

Gilligan P.H, Shapiro D.S and Miller M.B (2014). Cases in Medical Microbiology and Infectious Diseases. Third edition. American Society of Microbiology Press, USA.

Madigan M.T., Martinko J.M., Dunlap P.V and Clark D.P (2009). Brock Biology of Microorganisms, 12th edition. Pearson Benjamin Cummings Inc, USA.

Mahon C. R, Lehman D.C and Manuselis G (2011). Textbook of Diagnostic Microbiology. Fourth edition. Saunders Publishers, USA.

Patrick R. Murray, Ellen Jo Baron, James H. Jorgensen, Marie Louise Landry, Michael A. Pfaller (2007). Manual of Clinical Microbiology, 9th ed.: American Society for Microbiology.

Wilson B. A, Salyers A.A, Whitt D.D and Winkler M.E (2011). Bacterial Pathogenesis: A molecular Approach. Third edition. American Society of Microbiology Press, USA.

Woods GL and Washington JA (1995). The Clinician and the Microbiology Laboratory. Mandell GL, Bennett JE, Dolin R (eds): Principles and Practice of Infectious Diseases. 4th ed. Churchill Livingstone, New York.

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